Type 51B Luhai Class Modernisation

No guns that I am aware.   There are none from any of the mainstream ship accessory manufacturers, GMM, WEM, Toms, L'Arsenal.  A Chinese company, Veteran Models, made US weapon aftermarket items but they were IMO overpriced for what you got.  They have left the US market.  They may be an option.

WEM does make a 1:200 scale 130mm gun for the big Sovremennyy by Trumpeter,  but nothing in 1:350 scale for the subject.

The Z-9 is a Chinese license-built version of the Dauphin helo.   For that you are luckier,  L'Arsenal makes a replacement in 1:350 scale. 

Looking at the photo, the guns appear to be basically slab-sided with rounded edges.   That should be scratchbuildable with some sheet plastic and a set of scaled drawings.

Use a photocopier to enlarge the plans to the desired size.     Use the plans as templates to cut some sheet plastic.   I'd say 1mm thick.  Make a box shaped like the gun house.   Use some sanding sticks to round-over the edges of the box -- like the gun house shape.   Detail the outside with photoetch hatches (or thin plastic strip).  Use some wire to replicate the raised channel around the gun elevation slot.  

http://www.globalsecurity.org/military/world/china/luhai-pics.htm

Photos and plans are from Global Security dot org.   I don't know their original source.    Perhaps, check the US Navy's Naval Newsstand for pictures of foreign ship port visits.

The Navy Newsstand was a good idea. The photo happens to be cropped from the original, which was taken by Photographer's Mate 2nd Class Christopher S. Borgren II as she entered Apra Harbor, Guam.

This is the the best shot I could find of the turret. This turret is actually aboard the Type 053H3 Jiangwei II Class DDG 113 Qingdao. It's not truly on a horizontal plane but close enough. (photo by Mass Communications Specialist 3rd Class Ben A. Gonzales, Pearl Harbor)

After compensating for the angles from the photos I should be able to make a decent scratchbuilt turret. Thanks again, Ed. You've been a great help on this.